Some centrifugal separator designs employ a drum assembly which is spun at high speeds about a vertical rotational axis to cause the separation of constituents of different densities included in a feed stream introduced into the separator. In these designs, the drum assembly is spun about a vertical rotational axis as a feed stream is continuously introduced into a drum assembly volume defined by the drum assembly. Centrifugal force imparted on the feed stream by the rotation of the drum assembly causes higher-density constituents in the feed stream to collect at a maximum diameter region of the separator volume while lower-density constituents are displaced inwardly toward the axis of rotation. The lower-density constituents may exit the drum assembly volume via a lower-density material outlet at or near the axis of rotation at the top of the drum assembly volume. Higher-density material collecting in the region of maximum diameter within the drum assembly volume is ejected in a non-continuous fashion by periodically opening ejection passages formed in the drum assembly about the circumference of the drum assembly volume at the maximum diameter. A sliding piston mounted within the drum assembly volume is controlled to selectively open and close the drum ejection passages.
Among centrifugal separators of the type described in the previous paragraph there are generally two different methods used to remove the lower-density constituents from the drum assembly volume. In centrifugal separators commonly referred to as “non-hermetically sealed” separators, a centripetal pump may be used to pump collected lower-density material out of the drum assembly volume. In centrifugal separators commonly referred to as “hermetically sealed” separators, feed material is directed into the drum assembly volume so as to displace separated lower-density material without the need for a pumping element within the drum assembly volume. In either hermetically sealed or non-hermetically sealed centrifugal separators, the feed material may be introduced from the top of the drum assembly or from the bottom of the drum assembly.
In addition to removing higher-density constituents and lower-density constituents from a feed material, it may be desirable to also remove intermediate-density material which may collect radially inwardly from the higher-density material. For example, the intermediate-density material collecting radially inwardly of where the higher-density material collects may represent a product that is desirable to recover from the feed stream. In other cases, it may be desirable to remove the intermediate-density material from the drum assembly volume because the material interferes with the separation of the higher-density constituents of the feed stream from the lower-density constituents. In particular, the physical properties of the intermediate density material may be such that the material forms a barrier through which the higher-density material has difficulty passing even under the centrifugal force imparted by the rotation of the drum assembly.
This intermediate-density material may be removed by simply leaving the drum ejection passages open for a period of time longer than needed to eject the higher-density material. However, leaving the drum ejection passages open longer runs the risk of ejecting lower-density materials along with the higher-density materials and any intermediate-density materials. It may also be desirable to eject the intermediate-density material to facilitate separation but not eject the higher-density material.
In addition to or in lieu of periodically opened ejection passages, some centrifugal separators include specially sized orifices spaced apart at different angular orientations about the drum assembly axis of rotation. These orifices are continuously open to the drum assembly volume and are positioned and sized to allow collected material to exit the drum assembly volume at a desired rate.
Although such continuously open orifices may be used to eject intermediate-density material collecting at an intermediate region within the drum assembly volume, such orifices are difficult to size and position in practice so as to achieve the desired result. If the orifices are too large, excessive lower-density material will be ejected and thereby decrease the performance of the centrifugal separator. If the orifices are too small, intermediate-density material may continue to collect to interfere with the operation of the separator. Also, because the particular radius within the drum assembly volume where intermediate-density material may collect is somewhat dependent on the nature of the feed material, it is difficult to position orifices within the separator volume to remove all of the intermediate-density material in the operation of the centrifugal separator.
U.S. Pat. No. 9,561,513 shows a centrifugal separator having an arrangement for separating an input stream into a solid constituent, a heavy liquid phase, and a light liquid phase. The solid in this separator is ejected through ejection passages at the maximum diameter of the drum assembly volume, while the light liquid phase is removed via a centripetal pump as described above. The heavy liquid phase in the separator shown in U.S. Pat. No. 9,561,513 is removed through a channel that runs from an entry point at a location in the drum assembly volume inside the maximum diameter and then inwardly toward the center of rotation of the drum assembly. However, this arrangement requires that the heavy phase liquid move radially inwardly against the centrifugal force applied to the material in operation. This requirement that the heavy liquid phase move inwardly against the centrifugal force of the separator leaves the channel subject to plugging, which may be more or less severe depending upon the nature of the heavy liquid phase being separated.